1. Technical Field
The present disclosure relates to a separator having a modifier layer, a method for making the same, and a lithium ion battery including the separator.
2. Description of Related Art
A typical lithium ion battery mainly includes a cathode, an anode, a separator, and an electrolyte. The performance of electrode active materials of the cathode and anode is a key factor, which influences the performance of the lithium ion battery.
Typical cathode active materials are LiCoO2, LiNiO2, LiMn2O4, and LiFePO4. Typical anode active materials are carbonaceous materials such as graphite and carbon nanotubes. A conventional method for making an electrode includes steps of: mixing the electrode active materials, a conductive agent, and a binder as slurry; coating the slurry on a surface of a current collector, and heating the coated current collector to achieve the electrode. However, unwanted chemical reactions may occur during the charge and discharge of the lithium ion battery, especially at a high temperature. For example, some substance of the electrolyte may corrode the electrode active material or current collector such as aluminum foil. The separator may have an unacceptable shrinkage or may fuse at high temperatures. Thus, the lithium battery may have a low stability, and a capacity lost during the cycling process, especially at the high temperatures.
AlPO4 has been studied as a material to improve the thermal stability of the lithium ion battery (referring to the article “Correlation between AlPO4 nanoparticle coating thickness on LiCoO2 cathode and thermal stability. J. Cho, Electrochimica Acta 48 (2003) 2807-2811”).
In this article, a dispersion of AlPO4 particles dispersed in the water is prepared first, and then LiCoO2 cathode particles are added into the dispersion. Referring to FIG. 10, the AlPO4 particles 602 are adhered to the surface of the LiCoO2 cathode particles 604. After drying and heat treating processes, the LiCoO2 cathode particles 604 are coated with the AlPO4 particles 602. However, the small AlPO4 particles 602 agglomerate easily with each other in water because it is water insoluble. When many LiCoO2 cathode particles 604 are added in the dispersion, the first added LiCoO2 cathode particles 604 adhere to many AlPO4 particles 602, and subsequently added LiCoO2 cathode particles 604 may not adhere to enough AlPO4 particles 602. Even though the LiCoO2 cathode particles 604 are wholly coated, the AlPO4 particles 602 having uneven sizes are distributed on the surface of the cathode active material 604. Thus, the coated layer of AlPO4 particles 602 is non-uniform. Therefore, cycling performance of the lithium ion battery including this product 600 is not maximized. Further, the AlPO4 particle dispersion is difficult to use to improve other parts of the lithium battery, such as the separator and the current collector.
What is needed, therefore, is to provide a separator of the lithium ion battery and a method for making the same which when applied to the lithium ion battery can increase its stability and safety.